If you really have to leave during the meeting you’d better leave the back door.A. forB.

问题补充：

If you really have to leave during the meeting, you’d better leave the back door.

A. for

B. by

C. across

D. out

答案：

A. for

(答案→)B. by

C. across

D. out

解析：by表示“通过”，for 表示“目的”，across 表示“横穿”，out 表示“在外面”。